Hydrothermal carbonization

Hydrothermal carbonization (HTC) (also referred to as “aqueous carbonization at elevated temperature and pressure”) is a chemical process for the conversion of organic compounds to structured carbon. It can be used to make a wide variety of nanostructured carbons, simple production of brown coal substitute, synthesis gas , liquid petroleum precursors and humus from biomass with release of energy. The process, which technically imitates the brown coal formation (” coalification ” (in German)), which took place in 50,000 to 50 million years within a few hours, was investigated by Friedrich Bergius and first described in 1913.[1]

Motivation

The carbon efficiency of most processes is relatively low. Ie the proportion of carbon contained in the biomass, which is relatively low in the usable end product is relatively low:

process

Carbon efficiency

Alcoholic fermentation

67%

anaerobic conversion to biogas

50%

wood charcoal production

30%

production of humus by composting

5% to 10%

In the case of carbon dioxide, when fermented as methane. Both gases are considered to be climate-damaging. In addition, heat is released in these processes, which is used. Advanced modern system capture almost all the gases and use of the heat as part of the process or for district heating.

The problem with the production of biodiesel from oil plants is the only factor that the energy contained in the fruit can be used. If the whole plant could be used for fuel production, the energy yield could be increased by a growing cultivation area when growing fast-growing plants such as willow , poplar , miscanthus , hemp , reeds or forestry, with reducing energy, fertilizer and herbicide use, with the possibility of using – for current energy plant cultivation – poor soil. Hydrothermal carbonization makes it possible – similar to the biomass-to-liquid process – to use almost all of the carbon contained in the biomass for fuel generation. It is a new variation of an old field (biomass conversion to biofuel ) that has been developed in Germany. [2] It provides moderate temperatures and pressures over an aqueous solution of biomass in a dilute acid for several hours. The result is 100% of the carbonin a “biocoal” that could provide feedsource for soil amendment (similar to biochar ) and further studies in economic nanomaterial production. [3]

Process

Biomass is heated together with water at 180 ° C in a specific vessel, in particular vegetable material (in the following reaction equation, simplified as sugar C 6 H 12 O 6). The pressure rises to about 1 MPa (10 bar). During the reaction, oxonium ions are also formed which reduce the pH to 5 and lower. This step can be accelerated by adding a small amount of citric acid . [4]In this case, at low pH values, more carbon passes into the aqueous phase. The effluent reaction is exothermic, that is, energy is released. After 12 hours, the carbon of the reactants is completely reacted, 90 to 99% of the carbon is present as an aqueous sludge of porous brown coal spheres (C 6 H 2 O) with pore sizes between 8 and 20 nm as a solid phase The remaining 1 to 10% of carbon is either dissolved in the aqueous phase or converted to carbon dioxide. The reaction equation for the formation of brown coal is:

The reaction can be stopped in several stages with incomplete elimination of water, giving different intermediate products. After a few minutes, liquid lipophilic substances are formed, but their handling is very difficult because of their high reactivity. Subsequently, these polymerize substances and peat-like structures are formed, which are present as intermediates after about 8 hours.

Theoretically, the reaction with certain metal particles could be catalyzed , but these would be very quickly associated with their function.

Efficiency

As a result of the exothermic reaction of hydrothermal carbonization, about 3/8 of the calorific value of the biomass based on dry mass is released (with a high lignin , resin and / or oil content at least 1/4). If the process is managed properly, it is possible to use this waste heat to produce dry biocoal and to use some of the energy for energy generation.

In a large-scale technical implementation of hydrothermal carbonization of sewage sludge, it has been shown that about 20% of the energy content is contained in 90% end-dried. Furthermore, approximately 5% of the generated energy is necessary for the operation of the plant. It has proved to be particularly beneficial in the case of the process, with mechanical dehydration , more than 60% of the dry substance content can be achieved in the raw carbon, and thus the energy and equipment for the final drying of the coal is These are the methods of these slurries. [5]

Compared to sludge digestion with subsequent drying, the energy requirement of the HTC is less than 20% of the electrical energy and approximately 70% of the thermal energy. The amount of energy produced by the HTC has a stilable coal is simultaneously 10% higher. [6] Compared to conventional dry cleaning sludge, the HTC saves 62% of electricity and 69% of thermal energy due to its significantly simpler drainage. [7]

Benefits

An exothermic process would be advantageous in that the carbon content remains biologically, chemically or thermally convertible without further oxidation of the biomass. This could lead to a specific reduction in CO 2 release.

According to Markus Antonietti, the most important point is “… that one has a simple method of transforming atmospheric CO 2 via the detour of biomass into a stable and safe storage form, a carbon sink.” With hydrothermal carbonization, a large quantity of carbon can be stored decently all over the world. Liquid sequestration of carbon dioxide. With sufficient chemical stability of the coal, it could also be used for the improvement of soils (see also Terra preta).

The artificial humus could be used for the recycling of eroded surfaces. Due to the Increased plant growth in this way, additional carbon dioxide Could Be bound from the atmosphere so That a carbon efficiency of more than 1 gold negative CO 2 balance Could Be Achieved. The resulting carbon sludge could be used to burn fuel cells with a 60% efficiency, as currently being researched at Harvard University. In order to produce a conventional fuel, the carbon-water mixture would be more intensively, so that it is called synthesis gas , a gas mixture of carbon monoxide and hydrogen, is formed:

This synthesis gas could be used to produce gasoline via the Fischer-Tropsch process. Alternatively, the liquid intermediates that are formed during the incomplete conversion of the biomass could be used for fuel and plastic production.

In addition, the resulting carbon mud can be marketed to an environmentally friendly carbon dioxide-neutral “natural carbon” – Compared with the starting biomass, this can be dried to reduce the burden of energy and reduce the volume of storage.

An advantage of hydrothermal carbonization is that the usability of plant biomass is not restricted to plants with low moisture content and can not be obtained without carbon dioxide emission. For example, even scarcely usable plant material, such as waste from green areas, can be used to produce energy, [8] carbon dioxide is also being saved, which, together with more climate-damaging methane, would otherwise be produced by bacterial conversion of the biomass.

Problems

The major problem in the production of synthesis gas from biomass is the formation of tar , which can not be avoided during hydrothermal process management. However, it is not easy to see why this is the best way to biocarbon process. A biomass slurry should be decomposed into CO 2 and H 2 under supercritical conditions at 400 ° C and a pressure of at least 221.2 bar (critical temperature of water is 374 ° C), which requires a high energy input.

An appropriate process management as well as problems in the collection, transportation and storage of accumulated biomass are unresolved in this problem. These processes also require energy, which should be released by hydrothermal carbonation.

An advantage over dry thermal processes of refining biofuels with low moisture content is not so obvious. Even at the end of the 19th century, a weakly pyrolysed charcoal, which still contains at least 4/5 of the calorific value of the wood, was propagated for thermal processes.